This invention relates to an apparatus which allows a hydrostatic interface unit (HIU) to detect digital communication signals while minimizing the input power needed by the unit. More particularly, an active load within the hydrostatic interface unit provides a high impedance at AC frequencies for detecting digital communication signals, while providing a low impedance at DC frequencies, minimizing the input power needed for the hydrostatic interface unit.
The hydrostatic interface unit (HIU) is one of a series of instruments utilized in a tank gauging system. In this system, numerous liquid-filled tanks are located on a tank farm. Each tank has various transmitters connected to it. These transmitters are electrically coupled in parallel with one another in a "multidrop" arrangement and provide inputs to an HIU. The HIU accumulates data from various transmitters connected to it and combines their output representing pressure and temperature of the liquid in the tanks to provide a single output which indicates liquid level. The HIU outputs a digital signal to a control room via a translator. The control room monitors the liquid levels of numerous tanks and adjusts indicators, control outputs, or alarms as necessary.
In conventional HIUs, a resistor is used as a load across which communication signals appear and can be detected. In order to provide enough impedance at frequencies for detecting digital communication signals reliably, the resistor would have a minimum of 500 ohms of resistance. This 500 ohm resistor was sufficient for the HIU not adapted for use in so-called "designated field safe areas" where cabling is required to have specified low power levels to meet intrinsic safety requirements.
When a facility (such as a tank farm) uses or produces explosive vapors, special precautions must be taken with electrical circuits in areas of the facility where there are explosive mixtures. These areas are called "designated field safe areas" and are areas where the vapors can reach concentrations which may explode if there is an ignition spark. Also, some sparks have enough energy to ignite an explosive mixture and other sparks do not have enough energy to ignite an explosive mixture. Thus, electrical circuitry which enters "designated field safe areas" must have specified low voltage, low current, and energy storage capacity so that if its wires are inadvertently shorted out and a spark occurs, the spark will not have enough energy to ignite an explosive mixture. The limits have been developed experimentally and are defined in various safety standards. This is the subject of designing for intrinsic safety.
The problem of designing for intrinsic safety is magnified in a multidrop circuit, such as a tank gauging system having several transmitters connected to a single HIU, because the HIU and every transmitter connected to it must be energized within the power limit for one circuit. Small energy losses which normally would be ignored in other designs become extremely important and great efforts are utilized to reduce them. When small energy losses in a multidrop circuit are added together, the resulting amount of energy loss often exceeds the allowable limit for input energy to a HIU.
Thus, there is a need for a HIU with a load which will not only provide high impedances at AC frequencies for detecting digital communication signals, but will also provide low impedances at low frequencies (essentially DC), making intrinsic safety approval possible.